Assessment of empirical potential for MOX nuclear fuels and thermomechanical properties

Highlights

• Assessment of five empirical interatomic potentials for (U,Pu)O2.

• Comparison of the structural, thermodynamics, and mechanical properties.

• Studying crack propagation behaviour.

• Best results are obtained for the Cooper and Potashnikov potentials.

Abstract

We assess five empirical interatomic potentials in the approximation of rigid ions and pair interactions for the (U_1−y,Pu_y)O solid solution. The assessment compares available experimental data and Fink's recommendation with simulations on: the structural, thermodynamics, and mechanical properties over the full range of plutonium composition, from pure UO₂ to pure PuO₂ and for temperatures ranging from 300 K to the melting point. The best results are obtained by potentials referred as Cooper and Potashnikov potentials. The first one reproduces more accurately recommendations for the thermodynamics and mechanical properties exhibiting ductile-like behaviour during crack propagation, while the second one gives brittle behaviour at low temperature.

Introduction

Nowadays, uranium dioxide (UO₂) is being used as the standard nuclear fuel in fission nuclear reactors and has been extensively studied since the sixties. In parallel, nuclear fuel containing a mixture of uranium and plutonium oxides (MOX) as principle components provides an alternative, due to the fact that: (1) it allows large quantities of fissile isotopes produced in spent nuclear fuel from light water reactors to be recycled, (2) it can be seen as a more efficiently way of using the uranium dioxide, since the abundant ²³⁸U found in natural uranium is a Pu producer, (3) it can be taken as a solution for the increasing stockpile of Pu around the globe coming from either nuclear weapons and commercial reactors, and (4) it is designated as the most probable fuel for future fast breeder reactors, such as ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration).

A very important issue for the future of nuclear power is to ensure safeness and effectiveness during processes involving MOX fuel such as fabrication, operation and recycling. Yet, the toxicity of plutonium and high radiation levels make experiments less viable. Nonetheless, beside previous experimental efforts gathered in the following reviews [1], [2], experiments with MOX are difficult to perform, especially at high temperatures and under irradiation condition. For these reasons, numerical approaches can be chosen to bring some insight on basic physical phenomena that take place in the fuel matrix. For instance, over the last decade, several atomistic approaches using molecular dynamics (MD) simulations have been carried out to study thermal conductivity properties in (U,Pu)O₂ [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. However, the reliability of the results depends exclusively on the choice of the set of potentials. The potential parameters are usually fitted to reproduce a few physical properties, typically the lattice parameter, the cohesive energy, and complementary the elastic constants, which comes from experimental values or if not available from ab initio calculations. Therefore, each potential has its domain of validity. Subsequently, others physical properties for which the set of parameters are not been fitted need to be assessed to provide a good insight of advantages and disadvantages of each potential and their range of validity. This type of study has already been carried out in the case of UO₂ [13], [14], [15], [16] but, to our knowledge, not yet for MOX. Therefore, in this study, we assess available rigid ion model empirical potentials for MOX on the structural, thermodynamics, and mechanical properties. The assessment is performed over the full range of plutonium composition, from pure UO₂ to pure PuO₂ and for temperatures ranging from 300 K to melting point.

With MD method, actinide atoms are usually simulated in the approximation of rigid ions and pair interactions. For the mixed oxide compound (U,Pu)O₂, several interatomic potentials are available in the literature. There exists two main families of potentials. One that considers U and Pu cations as one single entity and hence they include only three set of parameters (A-A, A-O, and O-O) but depends on the relative percentage of Pu in the MOX [17]. The second one treats explicitly the U and Pu cations. Therefore, they include six set of parameters (U-U, U-Pu, Pu-Pu, U-O, Pu-O, and O-O) and do not depend on the percentage of Pu. Because we are interested in studying the spatial repartition of both cation and anion sublattices, we will only consider and describe the second type of force field.

Five rigid ion model potentials have been found in the literature and are tested herein. They will be coined by the name of the first author: Yamada [3], Arima [6], Potashnikov [15], Tiwary [18], and Cooper [19], [20]. We will present first the method used, then we will discuss the results obtained for the lattice parameter, the thermal expansion, the specific heat capacity, the elastic constants, the stress-strain curves under uniaxial deformation, and crack propagation.